1. Field of the Invention
This invention relates to improvements in contacts for making connection to semiconductor material, and, more particulary to a semiconductor contact system which provides electrical contact to a semiconductor region with control of the boundary recombination velocity to optimize semiconductor transport phenomena.
2. Description of the Prior Art
The termination and contact to a crystalline semiconductor have significant effects on the boundary recombination velocity of the semiconductor, which, in turn, significantly influences its transport phenomena. Contact to a semiconductor is usually realized by a metal to transport mobile carriers into or out of the semiconductor lattice in an exchange with an electrical ambient. Current flow can be viewed as an apparent flux of mobile carriers of one polarity traveling in one direction and an apparent flux of mobile carriers of the opposite polarity traveling in the opposite direction. With respect to an ammeter reading of the current, it is not possible to ascertain the relative flux stength of the respective mobile carriers. Although of little external importance, the relative flux weighting is of extreme importance to internal semiconductor transport phenomena.
Metals and semiconductors are different, since metals employ unipolar charge transport and semiconductors employ bipolar charge transport. Thus, in establishing a connection between a metal and a semiconductor, the metal-semiconductor interface requires a unipolar-bipolar charge conversion.
More particularly, at absolute zero a semiconductor has a filled valence band and an empty conduction band. As the temperature is increased, a number of mobile electrons are excited into the conduction band, leaving an equal number of holes in the valence band. Doping impurities introduce mobile carriers into one band, but not the other, thereby altering the effective bandgap energy.
The "chemostatic potential" of a material is a measure of its relative bipolar charge transport. For a zero chemostatic potential, the transport is purely bipolar, having equal mobile holes and electrons in their respective bands. For large chemostatic potentials, transport is effectively unipolar, a large positive chemostatic potential having primarily electron transport and a large negative chemostatic potential having primarily hole transport. One characteristic of semiconductor transport is that changes in the band-to-band recombination generation rate alter the chemostatic potential.
Metals, on the other hand, have one band completely filled, and another band partially filled, with no band-to-band mobile carrier transfer. Consequently, charge transport occurs via the partially filled band, without change in the the mobile carrier concentration. Thus, a metal acts as a "mobile carrier gas", with a constant chemostatic potential. For efficient energy exchange with a semiconductor, it is important that the metal mobile carrier gas have an energy associated with its partially filled band comparable to that of the semiconductor valence, or conduction, band. If the partially filled band of the metal is near the valence band of the semiconductor, the metal is termed a "hole-gas" metal, and if the partially filled band of the metal is near the conduction band of the semiconductor, the metal is termed an "electron-gas" metal.